Proteins have been labeled with various radiometals and other radioisotopic elements for use in immunodiagnostic and immunotherapeutic procedures. Some radiometals have superior properties for use in these techniques. Technetium-99m is an ideal radionuclide for scintigraphic imaging because of its nuclear properties. It has a single photon energy of 140 keV, a half-life of about 6 hours, and it is readily available from a .sup.99 Mo-.sup.99m Tc generator. Rhenium radioisotopes are beta-emitters which can kill target cells and thus are useful in therapy. Rhenium-186 and -188 also have gamma emission which, as an added feature, allows it to be imaged by scintigraphic techniques.
Two general approaches have been taken to label proteins such as antibodies with radiometals. The first is the direct labeling method by which the radiometal is bound to the protein molecule itself. The second is the indirect labeling method in which a chelating agent is coupled to the protein and the radiometal is attached to the protein via the chelating agent.
Rhodes discloses a method of direct labeling of protein with technetium-99m which involves ligand solid phase exchange. See U.S. Pat. No. 4,305,922. According to the method of Rhodes, pertechnetate is reduced to technetium IV and then applied onto a Sephadex.sup.R column. The reduced technetium-99m binds to the Sephadex.sup.R material. A solution of the protein to be labeled is poured onto the top of the Sephadex column where it is allowed to remain so that ligand exchange occurs. As a result, the technetium-99m is transferred preferentially from the Sephadex material to the protein. The protein may be pretreated with a stannous chloride (a procedure called "pretinning") to enhance transfer of the radiometal to the protein. See U.S. Pat. No. 4,424,200.
Various attempts have been made to label proteins with radiometals by the indirect approach. In one such approach, a chelating agent such a diethylenetriaminepentaacetic acid (DTPA) is conjugated onto the protein and then the metal ion is labeled onto the chelating agent attached to the protein molecule. For example, Khaw et al., Science 209: 295-297 (1980) discloses antibodies to cardiac myosin labeled with indium-111 via DTPA and use of the labeled antibodies to image for myocardial infarction. See also, Krejcarek et al., Biochem. Biophys. Res. Commun. 77: 581-585 (1977); Childs, R. L. and Hnatowich, D. J., J. Nucl. Med. 26: 293 (1985). In a more recent approach, Fritzberg et al. describe the use particular diaminodithiol and diamidodithiol groups, as a chelating agents. Fritzberg et al, J. Nucl. Med. 27:957 (1986); European Patent Application 86100360.6.
Various degrees of success have been achieved with both the direct and indirect methods of labeling proteins with radiometals. However, the labeled product is often unstable in vivo. Further, techniques for purifying the labeled product before use are often required. A need exists for improved methods for stably labeling proteins for radioimmunodiagnostic and radioimmunotherapeutic procedures.